NMR Studies of Hindered Rotation. The Diels-Alder Adduct of Phencyclone withp-Benzoquinone: Restricted Motion of Bridgehead Phenyls

Abstract

Abstract The Diels-Alder adduct of phencyclone with p-benzoquinone has been examined by 1H NMR at 300 MHz in CDCl3 at ambient temperatures for evidence of hindered rotation of the unsubstituted bridgehead phenyl groups. The nearly first-order spectrum exhibits four approximate doublets and five approximate triplets in the aromatic region, of roughly equal intensity (ca. 2H). This is consistent with a slow-exchange limit (SEL) spectrum of the hindered phenyls. For rapidly rotating phenyls, the predicted fast-exchange limit (FEL) spectrum would have shown two 2H doublets and one 4H doublet, in addition to three 2H triplets and one 4H triplet, in the aryl region. Full 1H assignments have been made, based on the two-dimensional 1H-1H homonuclear chemical shift correlation spectrum (COSY) and expected magnetic anisotropy effects.